Five-year-old Soren Chilson died early Tuesday morning at Springs Memorial Hospital in Lancaster.
Sheriff's deputies have arrested her mother's boyfriend, and charged him with killing her.
Phillip Bryan Gleason, 27, is charged with homicide by child abuse.
The arrest warrant obtained by Channel 9 says that Gleason struck the little girl in the head, and she died from blunt force trauma.
"It's an awful thing. Any time you have something like this involving a child," said Lancaster County Sheriff Barry Faile.
Deputies said Gleason was watching the girl Monday night at their home on Craig Farm Road while her mother was at work.
At some point after midnight he called 911. Chilson was found unresponsive on the bathroom floor of the house.
On Tuesday morning, Channel 9 found the small brick house surrounded by crime scene tape.
Investigators were taking items from the house, and taking pictures of the entire property.
Neighbors didn't know what had happened until our news crew told them.
"I don't have words. I don't know. It just makes me sick," said Paris Caskey. He said he rarely saw the family, and doesn't recall ever seeing the child outside. Barbara Black lives across the street and saw all the police cruisers outside early in the morning.
"It's tragic. For it to be a little girl like that. It's just sad," she said. "I thought they were a nice, happy family," she said.
Deputies told Channel 9 the couple had no history domestic violence or child abuse, and they weren't aware of previous calls to the house.
They believe Gleason had been in a relationship with the child's mother for only about six months.
Sheriff Faile said Gleason has not been cooperative with the investigation. He said detectives are still investigating, and won't comment about the potential for more arrests.
Gleason won't have a bond hearing until he can go before a circuit court judge. If convicted of homicide by child abuse, he could spend 20 years to life in prison, Faile said.
Gleason is a Winthrop University student in his second semester studying political science, the AP reported.